1. Field of the Invention
The present invention relates to an apparatus for blasting a surface and, more particularly, to a centrifugal blasting apparatus that is configured to deliver friable media without destroying a large percentage of the delivered media.
2. Description of the Related Art
It is often desirable to clean a surface by hurling small particles of blast media against the surface, such as to remove paint, rust and/or other coatings or built-up debris. In other situations, blasting the surface of an article may impart desirable qualities to the surface. One such situation in which this occurs is in the aerospace industry, where it has been found that blasting a surface of an article will place the surface of the article in compressive stress. This has been found to reduce the likelihood that the surface will crack or otherwise degrade when undergoing cyclic loading. Stressing the surface of an article by blasting the surface is referred to as peening. The term "blast" and "blasting" will be used herein generically to refer to any application in which small particles are hurled at a surface at a relatively high rate of speed. Exemplary applications include cleaning, descaling, deburring, deflashing, peening, etching, product appearance enhancement and numerous other similar applications.
There are two main types of devices that can be used for blasting. One common system is known as an air blast system. In an air blast system, a stream of compressed air carrying the blast media is released through a fixed nozzle, or manipulated by an operator or robotic device, and allowed to impinge a work surface. Although air blast systems are widely used, one drawback to using an air blast system is that the effective work area for the system is relatively small. For example, a conventional air blast system having a 3/8 inch diameter nozzle fed by a 30 HP compressor may propel approximately 30 pounds of media per minute, with an effective work surface area of about 2 in.sup.2. Air blast systems also are relatively noisy and require large or powerful air compressors.
Another type of device that can be used for blasting is a centrifugal blast system. In a centrifugal blast system, a spinning wheel is used to accelerate the blast media. Centrifugal blast systems are capable of delivering much more blast media over a much larger area than a comparable air blast system, while using less power and generating less noise. In a typical centrifugal blast system, the blast media enters a spinning wheel (referred to herein as a blast wheel) at a central location and is radially accelerated by centrifugal force toward the outside of the blast wheel. The blast wheel is typically provided with several similarly configured radially mounted blades, or vanes, that serve to channel and accelerate the blast media. The exit velocity of the particles of blast media leaving the blast wheel may be adjusted, inter alia, by adjusting the size of the blast wheel or by adjusting the rotational velocity of the blast wheel.
Many blasting applications use metallic particles as blast media. However, where ferrous contamination is undesirable or unacceptable or a particular surface finish is required, such as in the automotive, die casting and aerospace industries, metallic media typically cannot be used. In these applications, non-metallic media must be used, such as glass beads, ceramic beads, plastic beads, agri-shell, and baking soda. Likewise, it may be desirable to replace metallic media with softer non-metallic media for certain applications, such as removing paint and coatings while preserving the condition of the underlying surface. Since many non-metallic blast media are breakable, non-metallic media will be referred to hereinafter "friable."
Unfortunately, when a friable blast media is used with a conventional centrifugal blast apparatus, a large percentage of the media are destroyed. For example, it has been found that up to approximately 50% of the friable media is destroyed in one cycle through a conventional centrifugal blast apparatus. Since typical centrifugal blast systems recover and recycle the blast media, destruction of blast media significantly increases the cost of operation of the system. Accordingly, what is needed is a centrifugal blasting apparatus that is configured to deliver friable media with minimal destruction of the delivered media.